This invention relates to pressurized fluid supply systems which use a plurality of variable displacement pumps to provide operating fluid to a plurality of fluid-operated devices.
Fluid-operated devices, such as hydraulic actuators or cylinders for example, are usually operated with pressurized fluid supplied through a control valve from a pump driven by a motor. In most systems of this kind means must be provided to limit the output pressure of the pump both to avoid overtaxing the power output capabilities of the drive motor and to avoid excess leakage, hose or seal rupture and other adverse effects of an overpressure.
If the pump is of the fixed positive displacement type, a relief valve is connected between the pump output and the fluid reservoir to limit output pressure and to avoid lugging down and possible stalling of the drive motor. At such times as high-pressure fluid is being released back to the reservoir through the relief valve, a power wastage is occurring in that the motor must do work simply to force fluid through the relief valve back to the reservoir.
In order to reduce the power wastage which occurs when fluid is returned to tank through a relief valve, many pressurized-fluid supply systems use variable displacement pumps. Vairable displacement pumps have a control element which may be adjusted to change the amount of fluid translated through the pump during each pump shaft revolution between a maximum value and a minimum value. Feedback or servo mechanisms, commonly known as pressure compensators, are provided which sense output pressure and which function to increase displacement when output pressures are low and to reduce displacement, down to zero if necessary, when output pressures rise above a predetermined maximum value. System overpressures and possible stalling of the drive motor are avoided without relying on a continual pumping of a sizable fluid flow from the tank back to the tank through a relief valve.
As heretofore, constructed, pressure compensator arrangements have resulted in considerable complication of the structure of the system with adverse effects on bulk, weight and costs.
Certain pumps of this general type that have asymmetrical porting configurations are subject to an effect known as swivel torque force which tends to shift the pump towards the zero displacement position with a force which increases in magnitude as discharge pressure rises. Part of the complication and size of pressure compensator means in prior pumps of this type has resulted from arrangements designed to counteract this effect. It has not heretofore been recognized that swivel torque force may be advantageously utilized to aid in the pressure-limiting function.
Further problems are present in prior systems where a single drive motor operates a plurality of variable displacement pumps each of which supplies fluid to a separate driven device through a separate control valve. This is a common situation in various earthworking vehicles, for example, in which a single motor may drive several different pumps each associated with different fluid cylinders which operate different implements on the vehicle. A conventional pressure compensator at each pump avoids overpressures in the output of that particular pump and avoids overtaxing the drive motor insofar as the output power from that particular pump is concerned, but still further system complication has been needed to assure that the totalized loads of all of the pumps at any given time do not collectively overtax the driving motor.
To avoid overloading of the drive motor, the common practice has been to provide a summing valve receiving a pressure signal from the output of each of the pumps in the system and which acts to synchronously reduce the displacement of all of the pumps if the totalized output pressures exceed a predetermined maximum value. Aside from the structural complication involved, the action of a summing valve interferes with most efficient use of the system. In particular, the conventional arrangements cause the displacements of all pumps in the system to be decreased synchronously as the totalized output pressure from all pumps rises. All of the pumps in such a system are constrained to operate at the same power output level although the power requirements of the several fluid-operated devices associated with the several pumps may vary considerably.
If a particular pump experiences a relatively heavy loading, it would be preferable in many systems that it be able to operate at a higher power output level than the other pumps while the displacements of the other pumps are reduced, if necessary, to avoid overloading of the drive motor.